She went on to dismiss reports that her younger son, Dzhokhar, had already spoken about how the brothers planned the attack, questioning the veracity of news reports.

The mother spoke at an emotional news conference here alongside her husband. They grieved for their sons, but were at times visibly angry, pounding the table as they demanded more evidence that their sons were guilty, as well as proof that their elder son, Tamerlan, was not killed by police after being captured alive.

The couple cited videos they had seen on the Internet that claim to show Tamerlan being detained by police, followed by a gruesome photo of a naked dead body.

The father said he planned to travel soon to Boston to collect Tamerlan's body. He said they do not yet know whether they will bury him here or in the United States.

The parents also expressed frustration that they had been told they would not be allowed to visit Dzhokhar, the younger brother who was captured alive.

They said Dzhokhar, who suffered severe neck wounds, was being fed through a tube.

The mother, meanwhile, plans to remain in Dagestan for now. She denied the decision to stay behind was motivated by a fear that she could be arrested in the United States because of an outstanding warrant related to charges of shoplifting.

She called that incident "minor" and said that lawyers had secured some assurances that she would not be detained upon arrival.

The parents insisted that Tamerlan's trip to this region last year was only to visit family he had not seen in more than a decade and to attend a relative's wedding.

The father said it was impossible for Tamerlan to have made contact with extremists or militants during his six-month visit, saying his son never left his side during the three months they overlapped here.

Investigators want to know more about what Tamerlan did here during that time, concerned that he could have become radicalized in this restive region that is home to a militant Islamist uprising.

The FBI interviewed the parents this week in an effort to learn more about their sons and to inquire about Tamerlan's visit to Dagestan.

It was not the first time the parents had been questioned about Tamerlan by the FBI.

In 2011, the bureau, acting on a request from the Russian government, interviewed Tamerlan about possible links to extremists and militants abroad. In a statement Friday, the FBI said it found nothing suspicious at the time.

Tamerlan's parents, meanwhile, said his visit was a peaceful one. They say he enjoyed re-discovering the region where he lived for a few months as young teenager, visiting the beach and getting to know long-lost relatives.

The father said they attended prayers at several mosques around town, including the main mosque in downtown Makhachkala, as well as others, like a Salafist mosque on Katrova Street, that has drawn the eye of authorities.

The Imam there told ABC News Wednesday that they had never seen Tamerlan there, but noted that they have many people who attend prayers, with the faithful often forced to pray out in the street for lack of space.

The mother denied the mosque was as radical as some have claimed, asking why, then, it was still allowed to operate.

The mother said she never detected signs that her son had become radical.

She denied reports that a mysterious man named Misha, an Armenian convert to Islam, had radicalized Tamerlan.

She described Misha, whose last name and identity remain a mystery, as "the nicest man."

She said Tamerlan had befriended him at the mosque and he came to their house at least twice. While there, Misha showed them how to pray and the mother said that she was ashamed a convert knew Islamic traditions better than she did.

After Misha left, she said they were in awe of him and sought to follow his example.